The present invention relates to an optical circuit element and, more particularly, to an optical isolator which permits light to transmit in a predetermined direction only in an optical communication equipment, optical data equipment and similar equipment.
An optical isolator-typically comprises a Faraday rotator applied with a magnetic field capable of rotating the polarizing plane of incident light by 45 degrees, and two polarizers sandwiching the Faraday rotator from both sides, i.e., from the light incident side and from the light emerging side. Such an optical isolator exhibits a low loss in the forward direction in which light travels from the incident side to the emerging side, and exhibits a large loss in the reverse direction in which light returns from the emerging side back to the incident side. Thus, the optical isolator exhibits a function for transmitting light in one direction only, shutting off the transmission of light in the reverse direction.
Optical isolators of various types have heretofore been known. For instance, Japanese Examined Utility Model Publication (Kokoku) No. 7-17057 discloses an optical isolator constituted by a Faraday rotator which is a parallelepiped having an end surface on the incident side tilted relative to the optical axis and rotates, by 45 degrees, the polarizing plane of emerging light of a semiconductor laser, and a polarizing/isolating film formed on the end surface on the emerging side of the Faraday rotator. The polarizing/isolating film exhibits a polarizing/isolating property which permits the P waves to pass through but reflects the S waves, and can be formed by alternatingly laminating dielectric films having different refractive indexes and having a predetermined thickness one upon the other.
In the conventional optical isolators, the polarizer is made up of the polarizing/isolating film formed by laminating dielectric layers, as well as a polarized beam splitter, a polarizing glass, and a birefringent single crystal. However, these polarizers are all made from expensive starting materials, and require working steps such as cutting, polishing, etc., and are inevitably accompanied by complex production steps and increased production costs. In fact, the cost of producing the polarizer is about 50% of the total cost for manufacturing the conventional optical isolator. It has therefore been urged to lower the cost of producing the polarizer in the production of optical isolators.
One of the objects of the present invention is to provide an optical isolator which can be easily produced, while maintaining a good yield and a low cost, yet exhibiting an insertion loss and a reverse loss comparable to those of the conventional optical isolators.
In one embodiment thereof, the present invention relates to an optical isolator comprising the following elements in the described order:
a first reflection-type polarizer made of a polymeric multilayer film;
a Faraday rotator capable of rotating a polarizing plane by 45 degrees;
a tapered glass plate; and
a second reflection-type polarizer made of a polymeric multi-layer film.
In another embodiment thereof, the present invention resides in an optical isolator comprising the following constituent elements in the described order:
a first reflection-type polarizer made of a polymeric multilayer film;
a tapered Faraday rotator; and
a second reflection-type polarizer made of a polymeric multi-layer film.
In these optical isolator embodiments of the present invention, the constituent elements are preferably joined and integrally secured together with an adhesive or similar joining means.